Buried conductors

ABSTRACT

Buried conductors within semiconductor devices and structures, and methods for forming such conductors, are disclosed. In one embodiment of the invention, a semiconductor structure includes a substrate and a plurality of conductive elements buried within the substrate. The conductive elements may be metal, such as tungsten or a tungsten alloy. The invention described in the disclosure provides for advantages including formation of three-dimensional structures without resort to external wiring.

RELATED APPLICATION

This application is related to cofiled, copending and coassignedapplication entitled “Hi Q Inductive Elements”, U.S. application Ser.No. 9/069,346, now U.S. Pat. No. 6,025,261.

FIELD OF THE INVENTION

This invention relates generally to semiconductor technology, and moreparticularly to buried conductors within semiconductor devices andstructures, and methods for forming such conductors.

BACKGROUND OF THE INVENTION

Semiconductor technology pervades most electronic devices today.Computers, televisions, videocassette recorders, cameras, etc., all usesemiconductor integrated circuits to varying degrees. For example, thetypical computer includes microprocessors and dedicated controllerintegrated circuits (i.e., video controllers, audio controllers, etc.),as well as memory, such as dynamic random-access memory.

Traditionally, the semiconductor industry has used a process thatrequires first building a device or series of devices, and thenconnecting the devices together with a wiring level or levels above theactive-device surfaces. As integrated circuit designs have become morecomplex, several external wiring planes have been used to connectadjacent devices. As the number of wiring levels grew, the requirementof the vertical connections has significantly reduced the usefulness ofthe lower levels.

A limited prior art solution is to use a single buried wiring level inthe initial phases of semiconductor fabrication. A single buried wiringlevel overcomes the limitations of using external wiring planes, but isnot entirely applicable to current semiconductor technology. Forexample, stacked chip arrays require more than one wiring level.Therefore, even if a single buried wiring level is used, external wiringplanes are still nevertheless required.

For these and other reasons, there is a need for multiple buried wiringlevels within a single semiconductor device or structure.

SUMMARY OF THE INVENTION

The above-identified disadvantages, shortcomings and problems areovercome by the present invention, which will be understood by readingand studying the following specification. In one embodiment of theinvention, a semiconductor structure includes a substrate and aplurality of conductive elements buried within the substrate. Theconductive elements may be metal, such as tungsten or a tungsten alloy.

The use of a plurality of conductive elements buried within thesubstrate of a semiconductor structure overcomes limitations found inthe prior art. For example, where each conductive element is a separateburied plane, there may be two or more such planes, such that athree-dimensional semiconductor structure may be formed.Three-dimensional structures, as those of ordinary skill within the artcan appreciate, have the ability to significantly reduce the averagedistance from one segment of a large computer processor and associatedmemory to another, allowing for higher-density semiconductor storagedevices and faster processors.

The invention includes semiconductor structures, devices, and methods toform such structures and devices, of varying scope. In addition to theaspects and advantages of the present invention described in thissummary, further aspects and advantages of the invention will becomeapparent by reference to the drawings and by reading the detaileddescription that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a), 1(b) and 1(c) show exemplary semiconductor structuresaccording to varying embodiments of the invention;

FIGS. 2(f) and 2(g) show cross-sectional views respectively takengenerally along lines 2(f)-2(f) and 2(g)-2(g) of FIG. 2(a).

FIGS. 3(a), 3(b), 3(c), 3(d) and 3(e) show a process to form buriedconductors according to an embodiment of the invention; and,

FIG. 4 shows a representative semiconductor memory device in conjunctionwith which embodiments of the invention may be practiced.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of exemplary embodiments of theinvention, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration specificexemplary embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that logical, mechanical,electrical and other changes may be made without departing from thespirit or scope of the present invention. The following detaileddescription is, therefore, not to be taken in a limiting sense, and thescope of the present invention is defined only by the appended claims.

The detailed description is divided into five sections. In the firstsection, exemplary structures having buried conductors are described. Inthe second section, a method by which buried conductors may be formedaccording to one embodiment of the invention is provided. In the thirdsection, processes to form the exemplary structures of the first sectionare given. In the fourth section, a representative memory device inconjunction with which embodiments of the invention may be practiced ispresented. Finally, in the fifth section, a conclusion is provided.

Exemplary Structures

In this section of the detailed description, exemplary structuresaccording to varying embodiments of the invention are presented. Thoseof ordinary skill within the art will appreciate that the structuresdescribed in this section are merely exemplary, and the invention is notlimited to these particular structures. Rather, a description of theexemplary structures is provided to inform those of ordinary skillwithin the art the types of structures covered by the invention. Thestructures may thus be utilized in the formation of a semiconductordevice incorporating the structures.

Referring first to FIG. 1(a), one such exemplary structure is shown.Within N+ substrate 100 (i.e., doped silicon) are two buried conductivelayers, layer 102 and layer 104. Layer 102 includes a series ofconductive lines separated by an insulative material, where theconductive lines are parallel to the view shown in FIG. 1(a), such thatonly one such line is seen. Layer 104 includes a series of conductivelines also separated by an insulative material, but where the conductivelines are perpendicular to the view shown in FIG. 1(a), such that twosuch lines are seen. Layer 102 may thus be described as oriented in an xplane, and layer 104 oriented in a y plane. A conductive element mayrefer to either a layer within a semiconductor structure, or aconductive line within such a layer.

The invention is not particularly limited to the type of materialutilized to form the buried conductive layers or elements. A metal, suchas tungsten or a tungsten alloy, is desirably used. Other metals includethe non-radioactive elements selected from groups IVB, VB, VIB, VIIB,and VIIIB of the periodic table, and alloys of such elements. Eachconductive element, be it a separate layer or a line within a layer, maybe of the same material, or a different material, as the otherconductive elements.

Insulative material 106 surrounds conductive layers 102 and 104, toprevent short circuiting. The invention is also not particularly limitedto the type of insulative material used. In one embodiment of theinvention, silicon dioxide, also known as oxide, is utilized. Otherinsulative materials amenable to the invention include nitride, andoxy-nitride.

Over the layer formed by insulative material 106, as shown in FIG. 1(a)is active semiconductor layer 108 (e.g., a P− epitaxial layer). Theactive circuitry of the semiconductor structure are formed within thislayer. Because layer 108 rests on an insulator layer (material 106),such active circuitry includes silicon-on-insulator (SOI), as knownwithin the art.

Optionally, a connection between layers 102 and 104 may also be made, orbetween either layer and active semiconductor layer 108, usingion-implanted nickel, as described in the coassigned and copendingapplication entitled “Implanted Conductor and Method of Making,” filedMar. 14, 1997, Ser. No. 08/818,230, which is hereby incorporated byreference, or using a conventional fill strategy as known within theart.

Those of ordinary skill within the art can also appreciate thatadditional conductive layers may also be formed. Thus, third, fourth,fifth, et seq., layers may be formed within the structure as necessary.These other layers may be positioned above or below the other layers,and may have lines oriented in a direction parallel to or orthogonal tothe directions in which the lines of the other layers are oriented. Allthe conductive layers provide for internal wiring within thesemiconductor structure, to allow three-dimensional structures to beformed.

Referring next to FIG. 1(b), another exemplary structure according to anembodiment of the invention is shown. Within N+ substrate 110 are twoburied conductive elements, lines 114 and lines 114. Insulative material116 surrounds each line, and also separates lines 114 from activesemiconductor layer 118. In addition, deep trench capacitor 120 cutsthrough active layer 118, insulative material 116, and within substrate110 between lines 114. The deep trench capacitor trench is filled withP+ silicon, and is surrounded by dielectric material 117. An alternativeembodiment to that of FIG. 1(b) might have an additional level of buriedelements, where the elements are located either in front of and/orbehind the plane of the trench capacitor.

Referring now to FIG. 1(c), still another exemplary structure accordingto an embodiment of the invention is shown. Within N+ substrate 122 aretwo buried conductive layers, layer 124 and 126. Each layer includes aseries of conductive lines. Layer 124 is oriented in an x plane(parallel to the view shown in FIG. 1(c)), and layer 126 is oriented ina y plane (perpendicular to the view shown in FIG. 1(c)). Insulativematerial 128 surrounds conductive layers 124 and 126. Activesemiconductor layer 130 lies above the layer formed by insulativematerial 128.

Conductive connection 132 is formed prior to the buried wiring, as thoseof ordinary skill within the art can appreciate. Conductive connection132 thus provides an external connection to layer 124; connection 132cuts through substrate 122 at the back (bottom) of the structure of FIG.1(c). Connection 132 may then allow the die of which the structure is apart to be attached electrically to another structure of another die, asdescribed in U.S. Pat. Nos. 5,270,261 and 5,202,754, which are herebyincorporated by reference.

Connection 132 is desirably formed by first etching a hole withinsubstrate 122, and then oxidizing the surface to provide for insulativematerial 128 around the hole. A conductive plug may then be deposited toform connection 132. A metal, such as tungsten or a tungsten alloy, maybe the material utilized to form the connection. The invention is not soparticularly limited, however.

Another exemplary structure according to an embodiment of the inventionis shown by reference to FIGS. 2(a)-2(e). Referring first to FIG. 2(a),within N+ substrate 200 are two buried conductive layers, layer 204 and206, each having a plurality of lines. Each layer is oriented in boththe x and y planes; that is, each has a plurality of lines oriented inboth the x and y planes. Insulative material 208 surrounds conductivelayers 204 and 206. Active semiconductor layer 210 lies above the layerformed by insulative material 208. As shown in FIG. 2(a), the parts ofeach layer 204 and 206 oriented in the x plane, parallel to the viewshown in FIG. 2(a), are accessible from the top of the structure. Thatis, a conductive connection to each of these parts of layers 204 and 206cuts through active layer 210 and the layer formed by insulativematerial 208.

Referring next to FIG. 2(b), a first mask pattern used to produce thetrenches for (first, lower) layer 206 of the structure is shown. Lines212 correspond to the lines of conductive layer 206 that are oriented inthe y plane, and lines 214 correspond to the lines of conductive layer206 that are oriented in the x plane. Exposure of the substrate layerusing this first mask pattern, and subsequent etching, thus results inthe formation of trenches which, when deposited with conductivematerial, make for the lines of layer 206, as those of ordinary skillwithin the art can appreciate.

Similarly, referring to FIG. 2(c), a second mask pattern used to producethe trenches for (second, higher) layer 204 of the structure is shown.Lines 216 correspond to the lines of conductive layer 204 that areoriented in the y plane, and lines 218 correspond to the lines ofconductive layer 204 that are oriented in the x plane. Exposure of thesubstrate layer, after formation of layer 204, using this second maskpattern, and subsequent etching, thus results in the formation oftrenches which, when deposited with conductive material, make for thelines of layer 204.

Finally, FIG. 2(d) shows a portion of the silicon island structure thatmay be used as the seed points for the epitaxial growth of the N+substrate level, while FIG. 2(e) shows a schematic layout of possiblecontacts to the buried wiring lines (both within layer 204 and layer206). The structure of FIGS. 2(a)-2(e) thus is a three-dimensionalsemiconductor structure, having two levels of buried wiring lines; eachlevel of buried wiring lines includes lines oriented in the x and the yplanes. Such buried wiring lines may render it unnecessary to useexternal wiring for connections of the structure.

Exemplary Method to Form Buried Conductors

In this section of the detailed description, an exemplary method to formburied conductors, according to one embodiment of the invention, isdescribed. Utilizing this method, those of ordinary skiff within the artwill then be able to form the exemplary structures of the previoussection.

Referring first to FIG. 3(a), within silicon substrate 300, trench 302,of the desired width and depth is cut. Trench 302 has a bottom, and atleast two sidewalls. Substrate 300 is part of a wafer having a frontside (top) and a back side (bottom). Referring next to FIG. 3(b),oxidation is performed to produce oxide layer 304 of sufficientthickness to provide electrical insulation for the subsequently formedwire. Oxide layer 304 is present on the bottom and the walls of trench302. Seed layer 306 is then deposited, desirably using a line of sightdeposition process such as collimated sputtering. In one embodiment,seed layer 306 is titanium, although the invention itself is not solimited to this material. The seed material may be an element selectedfrom groups IVB, VB, or VIB, of the periodic table, for example.

Referring next to FIG. 3(c), the seed layer and the oxide on the topsurface of the wafer (i.e., not at the bottom of trench 302) is removed,desirably by chemical mechanical polishing (CMP), or another selectiveremoval process. The conductive material to be used to form the buriedwiring level is then deposited within the trench, such as tungsten 308,substantially at the bottom of the trench (i.e., over the seed layer).This is desirably accomplished by a selective metal deposition process,such as plating or chemical vapor deposition. (However, a line of sightprocess—such as columnated sputtering or long through evaporation—mayalso be used, in which case a seed layer deposition prior to conductivematerial deposition is unnecessary.) Any incidental deposition on thesurface of the wafer may be removed by CMP. Insulative material 310 isthen deposited to sufficiently fill the trench. Material 310 may beoxide (silicon dioxide), or any other suitable insulator.

Referring next to FIG. 3(d), the top wafer surface is again polished byCMP or another suitable process to expose substrate 300, but not farenough to expose tungsten 308. The exposed substrate 300 may then beused as seed points for epitaxial silicon growth. It is noted that thewidth of tungsten 308 within trench 302 is less than the photo imagethat was used to etch trench 302; thus, it is possible to produce linewidths that are less than the minimum photo-definable image.

Referring finally to FIG. 3(e), the process of FIGS. 3(a) through 3(e)may be repeated to produce another level of buried wiring. Thus, asshown in FIG. 3(e), two buried conductors are present: a deeperconductor at the bottom of trench 302, and a shallower conductor at thebottom of trench 312. Greater-depth conductors are formed first, andthen lesser-depth conductors are formed. Utilizing the describedprocess, any number of buried wiring levels may be produced.Furthermore, while as described only one trench for each layer has beenformed, those of ordinary skill within the art can appreciate that anynumber of trenches for each layer may be formed.

Other steps may also be added to the described process. For example, theback side of the wafer may be thinned to expose parts of at least thebottom-most conductive layer. For further example, the first layer maybe connected to the second layer, using ion-implanted nitrogen, asdescribed in the coassigned and copending application entitled“Implanted Conductor and Method of Making,” filed Mar. 14, 1997, Ser.No. 08/818,230, or using a conventional fill strategy as known withinthe art.

Specific Processes

Utilizing the exemplary method described in the previous section of thedetailed description, those of ordinary skill in the art are thusenabled to form the exemplary structures described in the first sectionof the detailed description. However, specific processes to form theexemplary structures of the first section of the detailed descriptionare provided in this section of the detailed description. Theseprocesses correspond to the exemplary method described in the previoussection, except that more specific detail is provided.

A specific process that can be used to produce the structures of FIG.1(a) and FIG. 1(c) (except for the tungsten connection of FIG. 1(c), aprocess to form such a connection having already been described) isfirst provided. Starting with an N+ wafer, 0.3 micron width trenches 1.2microns deep are etched in the silicon to form the spaces for the first(deepest) level of wiring. 850 angstrom of thermal oxide are then grown,thus reducing the trench width to 0.225 micron. Alternatively, insteadof growing thermal oxide, 500 angstroms of CVD oxide may be deposited.The use of a CVD layer is particularly advantageous when several layersof buried wiring are to be used. 75 angstrom of titanium is thendeposited using collimated sputtering. Chemical mechanical polishing isperformed to remove titanium and oxide from the wafer surface, exposingthe N+ silicon underneath. Next, 0.4 micron of tungsten is selectivelydeposited over the titanium, using low-temperature hydrogen reduction oftungsten fluoride, where the titanium serves as the seed layer. 1450angstrom of silicon dioxide is deposited, and chemical mechanicalpolishing is used to remove the oxide from the wafer surface, butleaving oxide over the tungsten-filled trenches. Thus, the first wiringlevel is constructed.

A second wiring level is constructed by first etching 0.3 micron widthtrenches 0.6 micron deep in the silicon and the oxide where the trenchcrosses the oxide-covered tungsten of the first wiring level. 850angstrom silicon oxide is deposited, and 75 angstrom of titanium isdeposited. The titanium and oxide on the wafer surface is removed viachemical mechanical polishing. If connections are desired between thetwo levels of conductors, they can be made through the insulatorseparating the layers (or the conducting layer and an active device)using ion-implanted nickel, as described in the coassigned and copendingapplication entitled “Implanted Conductor and Method of Making,” filedMar. 14, 1997, Ser. No. 08/818,230, or using a conventional fillstrategy as known within the art. 0.4 micron of tungsten is selectivelydeposited over the titanium, and chemical mechanical polishing oranother suitable technique is used to remove any tungsten inadvertentlydeposited on the wafer surface. 5200 angstrom of oxide is deposited viachemical vapor deposition to fill the trenches and cap the conductors.

Finally, chemical mechanical polishing is used to remove the oxide andexpose the silicon surface. The open windows of the exposed N+ siliconare used as nucleation points to grow P− silicon epitaxially. A desiredphoto mask is used to define the areas of P−/N+ contact and toselectively remove the P− silicon. 1000 angstrom of oxide is deposited,and chemical mechanical polishing is used to expose the P− silicon.Finally, the P− areas that are exposed are used as nucleating sites togrow a P− epitaxial layer to cover the wafer surface.

A specific process that can be used to form the structure of FIG. 1(b)is now described. On an N+ wafer, 0.3 micron width trenches, 0.6 micronsdeep, are etched to form the spaces for the buried level of wiring. 850angstrom of thermal oxide is grown. 75 angstrom of titanium is depositedusing collimated sputtering, to provide a seed layer for the conductivematerial. Chemical mechanical polishing, or another suitable technique,is used to remove titanium and oxide from the wafer surface. 0.4 micronof tungsten is selectively deposited at the bottom of the trench, usinglow temperature hydrogen reduction of tungsten fluoride, where thetitanium serves as the seed layer. 1075 angstrom of oxide is deposited,and chemical mechanical polishing is used to remove the oxide from thewafer surface. The resulting open windows to the N+ silicon are used asnucleation points to grow a P− silicon epitaxial layer.

Next, an appropriate photomask is used to define the areas of P−/N+contact, and the deep trench capacitor is etched. A desiredoxygen-nitrogen-oxygen (ONO) structure, or other dielectric, is grown toprovide the capacitor dielectric. The capacitor trench is then filledwith P+ polysilicon, and chemical mechanical polishing is used to exposethe P+ silicon surface. Conventional processing may then be used to formdevices and memory cells within the active layer.

Finally, a specific process that can be used to produce the structure ofFIGS. 2(a)-2(e) is described. On an N-type wafer, 0.3 micron widthtrenches, 1.2 microns deep, are etched to form the spaces for the first(deepest) level of wiring. 850 angstrom of thermal oxide is grown, and75 angstrom of titanium is deposited using collimated sputtering.Chemical mechanical polishing is used to remove titanium and oxide fromthe wafer surface. 0.4 micron of tungsten is selectively deposited overthe titanium at the bottom of the trench using low temperature hydrogenreduction of tungsten fluoride, the titanium serving as the seed layer.1450 angstrom of oxide is deposited, and chemical mechanical polishingis used to remove the oxide from the wafer surface.

Next, 0.3 micron width troughs, 0.6 microns deep, are etched in thesilicon and the oxide in the pattern of the upper (second) level ofmetal. 850 angstrom of thermal oxide is CVD deposited and 75 angstrom oftitanium is deposited. Titanium and oxide from the wafer surface isremoved via chemical mechanical polishing. If connections are desiredbetween the two levels of conductors, they can be made through theinsulator separating the layers (or the conducting layer and an activedevice) using ion-implanted nickel, as described in the coassigned andcopending application entitled “Implanted Conductor and Method ofMaking,” filed Mar. 14, 1997, Ser. No. 08/818,230, or using aconventional fill strategy as known within the art. 0.4 microns oftungsten are selectively deposited over the titanium. If necessary, anytungsten inadvertently deposited on the wafer surface is removed usingchemical mechanical polishing or other suitable techniques. 5200angstrom of oxide is deposited via chemical vapor deposition, andchemical mechanical polishing is used to remove the oxide sufficientlyto expose the N+ silicon surface.

Next, using the open windows to the N+ silicon as nucleation points, P−silicon is grown epitaxially. If contacts from the buried wiring levelsto structures on or above the P− epitaxial surface are desired, windowsare etched in the epitaxial layer using a mask (such as that which hasbeen shown in FIG. 2(d)). An oxide layer equal in thickness to the P−silicon layer is then deposited, and the surface of the wafer ispolished (via chemical mechanical polishing) to expose the P− silicon;contact is then made to the wiring levels using ion-implanted nickel asdescribed in the coassigned and copending application entitled“Implanted Conductor and Method of Making,” filed Mar. 14, 1997, Ser.No. 08/818,230, or using a conventional fill strategy as known withinthe art.

Those of ordinary skill within the art can appreciate that the order ofthe processes described may be modified without affecting materially thefinal structure produced. For example, in the process just described,the oxide could be removed from the wafer surface prior to titaniumdeposition, with the titanium on the surface being removed in asubsequent polishing process. The seed layer used could be othermaterials besides titanium, such as zirconium. Similarly, while thematerial used for the wiring structure was described as tungsten, othermetals and alloys thereof are suitable. Desirably, a material ofsufficiently low resistance to provide a good electrical conductor isused. Sufficient electromigration resistance to enable the requiredcurrent to be carried without failure during the devices' expectedlifetime is desirable.

Furthermore, the diffusion rate of the element or elements used asconductors through the oxide or other insulator should desirably be lowenough so that the amount diffusing through the insulating film and intothe silicon is insufficient to degrade the properties of the device ordevices formed in the various silicon layers. The melting point of themetal or alloy used should desirably be sufficiently high to prevent itsmelting or other unwanted metallurgical changes during furtherprocessing of the silicon. For example, aluminum and aluminum alloysthat have been used in traditional wiring applications may have meltingpoints that are low enough to cause concern; however, future changes inprocessing technology that may lower the temperature requirements ofdevice operation may mute this concern.

In addition, copper, which has a high diffusion rate in silicon dioxideand is a junction “poison,” as known within the art, is another metalwhich at the time of filing of this patent application may not be ableto be considered for use unless a suitable barrier layer surrounds it.The requirements for the buried wiring levels may be fulfilled with moreor less ease by a number of materials, including the non-radioactiveelements of groups IVB, VB, VIB, VIIB, and VIIIB of the periodic table.Of these elements, those in groups VB, VIB and VIIIB may be mostdesirable because of their lower resistance. A material such as tungstenmay be desired because of its reasonably low resistance and because itsdeposition is relatively well understood by those of ordinary skillwithin the art.

Furthermore, although selective chemical vapor deposition has beenspecifically referenced to deposit the conductive material after theseed layer has been deposited, the invention is not so limited to suchdeposition. For example, electroless plating may be substituted forselective chemical vapor deposition. Also, instead of chemical vapordeposition oxide for the final trench fill after the last metaldeposition, an alternative material such as polyimide may also be used.

Representative Memory Device

In this section, a typical memory device in conjunction with which anembodiment of the invention may be implemented is described. Referringto FIG. 4, memory device 10 includes memory array 12, control circuit14, input/output buffers 16, and address decoder 18. Memory array 12includes a plurality of memory cells organized into rows and columns.The memory cells are accessed in response to an address signal providedon the address lines A0 . . . AN running to address decoder 18, whichincludes both row and column decoder circuitry. Input/output buffers 16are provided for bidirectional communication via the data communicationslines DQ1 . . . DQN running out of buffers 16. Control circuit 14regulates the memory device operations in response to control signalsincluding, but not limited to, a Clock (CLK), a Row Access Strobe(RAS*), a Column Access Strobe (CAS*), a Write Enable (WE*), and a ClockEnable (CKE).

As will be appreciated by those skilled in the art, the presentinvention is not limited to any particular type of memory device orrepair memory device. In one embodiment, the devices are dynamicrandom-access-memories (DRAMs), including those available from MicronTechnology, Inc., of Boise, Id. In other embodiments, the devices arestatic random-access-memories (SRAMs), flash memories, synchronousdynamic random-access-memories (SDRAMs), extended-data-outrandom-access-memories (EDO RAMs), and burst-extended-data-outrandom-access-memories (BEDO RAMs), as those skilled in the art willappreciate.

CONCLUSION

Buried conductors have been described. The utilization of suchconductors provides for internal wiring levels within semiconductorstructures and devices, making external wiring levels unnecessary. Thus,three-dimensional structures, such as stacked-chip arrays, may beconstructed without resorting to such external wiring levels. Thisrenders these structures faster, and more densely integrated, than wouldotherwise be possible. Those of ordinary skill within the art can,however, appreciate that other advantages may and can be provided by theinvention.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. This application isintended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the following claims and equivalents thereof

We claim:
 1. A semiconductor structure comprising: a substrate layerhaving an active semiconductor layer formed thereon; a first buriedlayer buried at a first depth within the substrate and consisting of aplurality of substantially parallel first conductive elements orientedin a first direction; a second buried layer buried at a second depthwithin the substrate and consisting of a plurality of substantiallyparallel second conductive elements oriented in a second directionorthogonal to the first direction, wherein the second depth is greaterthan the first depth; insulating material arranged to separate the firstand second buried layer and to separate the first and second conductiveelements, wherein the first buried layer and the second buried layer aresubstantially parallel to the active semiconductor layer; wherein theinsulating material separates the first buried layer from the activesemiconductor layer; wherein the first buried layer and the secondburied layer include a material with a high melting point that preventsunwanted metallurgical changes when processing the active semiconductorlayer; and wherein the plurality of substantially parallel secondconductive elements are connected to conductive parts that extendupwardly on opposite sides of the first buried layer beyond the firstdepth adjacent outer ones of the plurality of substantially parallelfirst conductive elements.
 2. The structure of claim 1, wherein at leastone of the first conductive elements and the second conductive elementsis a metal.
 3. The structure of claim 2, wherein the metal comprises oneof tungsten and a tungsten alloy.
 4. The structure of claim 1, whereinthe substrate comprises doped silicon.
 5. The structure of claim 1,wherein the insulating material is selected from the group consistingof: oxide, oxy-nitride, and nitride.
 6. A semiconductor structurecomprising: a substrate layer having an active semiconductor layerformed thereon; a first buried layer buried at a first depth within thesubstrate and consisting of a plurality of substantially parallel firstconductive elements oriented in a first direction and surrounded by aninsulating material; a second buried layer buried at a second depthwithin the substrate, separated from the first buried layer by theinsulating material, the second buried layer consisting of a pluralityof substantially parallel second conductive elements surrounded by theinsulating material and oriented in a second direction orthogonal to thefirst direction, wherein the second depth is different than the firstdepth; a connection formed between the first and second buried layers;wherein the first and second buried layers are substantially parallel tothe active semiconductor layer and include a material with a highmelting point that prevents unwanted metallurgical changes; and whereinthe plurality of substantially parallel conductive elements areconnected to conductive parts that extend upwardly on opposite sides ofthe first buried layer beyond the first depth adjacent outer ones of theplurality of substantially parallel first conductive elements.
 7. Thestructure of claim 6, wherein at least one of the first conductiveelements, the second conductive elements and the connector is metal. 8.The structure of claim 7, wherein the metal comprises one of tungstenand a tungsten alloy.
 9. The structure of claim 6 wherein the substratecomprises doped silicon.
 10. The structure of claim 6 wherein theinsulating material is selected from the group consisting of: oxide,oxy-nitride, and nitride.
 11. A semiconductor device comprising: asubstrate having an active semiconductor layer formed thereon; a firstburied layer buried at a first depth within the substrate and consistingof a plurality of substantially parallel first conductive elementsoriented in a first direction; a second buried layer buried at a seconddepth within the substrate and comprising a plurality of substantiallyparallel second conductive elements oriented in a second directionorthogonal to the first direction, wherein all of the first conductiveelements are not electrically connected to the second conductiveelements, wherein the second depth is different than the first depth; aconductive connection formed beneath one of the buried layers andarranged to provide an external connection to one of the buried layers;and wherein the plurality of substantially parallel second conductiveelements are connected to conductive parts that extend upwardly onopposite sides of the first buried layer beyond the first depth adjacentouter ones of the plurality of substantially parallel first conductiveelements.
 12. The device of claim 11 wherein at least one of the firstconductive elements, the second conductive elements and the connectionis a metal.
 13. The device of claim 12, wherein the metal comprises oneof tungsten and a tungsten alloy.
 14. The device of claim 11 wherein thesubstrate comprises doped silicon.
 15. The structure of claim 11 whereinthe insulating material is selected from the group consisting of: oxide,oxy-nitride, and nitride.
 16. The device of claim 11 wherein the firstand second buried layers are substantially parallel to the activesemiconductor layer.
 17. The semiconductor device of claim 11 whereinthe first buried layer includes a material with a high melting pointthat prevents unwanted metallurgical changes.
 18. A semiconductor devicecomprising: a substrate having an active semiconductor layer formedthereon; a first buried layer buried at a first depth within thesubstrate and comprising a plurality of substantially parallel firstconductive elements oriented in a first direction and a plurality ofsubstantially parallel second conductive elements oriented in a seconddirection orthogonal to the first direction, wherein the firstconductive elements are not electrically connected to any of the secondconductive elements; a second buried layer buried at a second depthwithin the substrate and comprising a plurality of substantiallyparallel third conductive elements oriented in a third direction and aplurality of substantially parallel fourth conductive elements orientedin a fourth direction orthogonal to the third direction, wherein thesecond depth is different than the first depth; and wherein theplurality of substantially parallel third conductive elements areconnected to conductive parts that extend upwardly on opposite sides ofthe first buried layer beyond the first depth adjacent outer ones of theplurality of substantially parallel first conductive elements.
 19. Thesemiconductor device of claim 18, wherein the first buried layer and thesecond buried layer are parallel.
 20. The semiconductor device of claim18, wherein the first buried layer and the second buried layer havesurfaces in non-intersecting planes.
 21. The semiconductor device ofclaim 18 wherein the first buried layer includes a material with a highmelting point that prevents unwanted metallurgical changes.
 22. Asemiconductor structure comprising: a substrate layer having an activesemiconductor layer formed thereon; a first buried layer buried at afirst depth within the substrate and consisting of a plurality ofsubstantially parallel first conductive elements oriented in a firstdirection, the plurality of first conductive elements each having asurface in a first plane; a second buried layer buried at a second depthwithin the substrate and consisting of a plurality of substantiallyparallel second conductive elements oriented in a second directionorthogonal to the first direction, the plurality of second conductiveelements each having a surface in a second plane, wherein the seconddepth is different than the first depth insulating material arranged toseparate the first plane and the second plane and to essentiallysurround the first and second conductive elements, wherein the firstplane and the second plane are substantially parallel to the activesemiconductor layer; and wherein the plurality of substantially parallelsecond conductive elements are connected to conductive parts that extendupwardly on opposite sides of the first buried layer beyond the firstdepth adjacent outer ones of the plurality of substantially parallelfirst conductive elements.
 23. The semiconductor device of claim 22wherein the first buried layer includes a material with a high meltingpoint that prevents unwanted metallurgical changes.
 24. A semiconductordevice comprising: substrate having an active semiconductor layer formedthereon; a first buried layer buried at a first depth within thesubstrate and comprising a plurality of substantially parallel firstconductive elements oriented in a first direction and a plurality ofsubstantially parallel second conductive elements oriented in a seconddirection orthogonal to the first direction, wherein all of the firstconductive elements are not electrically connected to the secondconductive elements, wherein the plurality of first conductive elementseach include a surface in a first plane; a second buried layer buried ata second depth within the substrate and comprising a plurality ofsubstantially parallel third conductive elements oriented in a thirddirection and a plurality of substantially parallel fourth conductiveelements oriented in a fourth direction orthogonal to the thirddirection, wherein the plurality of third conductive elements eachinclude a surface in a second plane, wherein the second depth isdifferent than the first depth; and wherein the plurality ofsubstantially parallel third conductive elements are connected toconductive parts that extend upwardly on opposite sides of the firstburied layer beyond the first plane adjacent outer ones of the pluralityof substantially parallel first conductive elements.
 25. Thesemiconductor device of claim 24, wherein the first and second planesare parallel to each other.
 26. The semiconductor device of claim 25,wherein the first plane is parallel to the active device region.
 27. Thesemiconductor device of claim 24, wherein the plurality of secondconductive elements each include a surface in the first plane.
 28. Thesemiconductor device of claim 27 wherein the plurality of fourthconductive elements each include a surface in the second plane.
 29. Thesemiconductor device of claim 24 wherein the plurality of fourthconductive elements each include a surface in the second plane.
 30. Thesemiconductor device of claim 24, wherein the first buried layerincludes a material with a high melting point that prevents unwantedmetallurgical changes.
 31. A semiconductor device comprising: asubstrate having an active semiconductor layer formed thereon; a firstburied layer buried at a first depth within the substrate and consistingof a plurality of substantially parallel first conductive elementsoriented in a first direction and a plurality of substantially parallelsecond conductive elements oriented in a second direction orthogonal tothe first direction, wherein all of the first conductive elements arenot electrically connected to the second conductive elements, whereinthe first buried layer includes a material with a high melting pointthat prevents unwanted metallurgical changes; a second buried layerburied at a second depth within the substrate and constituting of aplurality of substantially parallel third conductive elements orientedin a third direction and a plurality of substantially parallel fourthconductive elements oriented in a fourth direction orthogonal to thethird direction, wherein the second depth is different than the firstdepth; and wherein the plurality of substantially parallel thirdconductive elements are connected to conductive parts that extendupwardly on opposite sides of the first buried layer beyond the firstdepth adjacent outer ones of the plurality of substantially parallelfirst conductive elements.